1. Field of the Invention
The invention relates to multilayer thermoplastic composites made from a polyamide-based molding composition, a polyester-based molding composition and an adhesion promoter; and to a process for the production of these composites; and to their use.
2. Description of the Prior Art
Polyamides and polyesters on their own are unsuitable for many applications. Thus, polyamides are, for example, not resistant to weathering since they age on exposure to light and absorb moisture from the atmosphere. This results in discoloration, impairment of mechanical properties and warping.
Although polyamides on their own have good mechanical properties, in particular good toughness, they have a poor barrier action. Polar substances, in particular, can easily migrate through polyamides. This is extremely disadvantageous, for example in fuel lines transporting alcohol-containing fuel.
Polyesters generally have good weather resistance and have an excellent barrier action both to polar and to nonpolar media. However, they are generally impact-sensitive. The notched impact strength, in particular, in polyesters is frequently inadequate. The resistance to some chemical influences is also inadequate. Polyesters therefore cannot be used in many cases where other properties, such as their excellent barrier action, high temperature resistance and good rigidity, would actually be desired.
It would therefore be desirable if it were possible to produce a strong bond between polyamide and polyester. It would thus be possible, for example, to protect polyamide moldings against light and moisture by coating with polyester. Likewise, it would be possible to protect polyester moldings against chemical and mechanical influences by coating with polyamide. A further advantage here would be better printability.
Composites made from polyamide and polyester have, in principle, already been disclosed. EP-A 0 336 806 described the coextrusion of polyamide 12 (PA 12) and polybutylene terephthalate (PBT) to give a two-layer tube. German Patent 38 27 092 describes a multilayer tube which comprises, from the inside outward, layers of polyamide, polyvinyl alcohol, polyamide and polyester. However, it is known to a person skilled in the art that, by far, the majority of polymers, including polyamides and polyesters, are incompatible with one another, which is why no adhesion is achieved between the laminate layers in the production of polymer laminates. However, an adhesion-based bond between the individual polymer layers is absolutely necessary in conventional industrial applications.
EP-A 0 287 839 discloses composites made from specific polyamide mixtures and various other thermoplastics, such as, for example, polyethylene terephthalate. In order to achieve the requisite adhesion between the two layers, an adhesion promoter is introduced between the laminate layers. Suitable adhesion promoters indicated in this publication are functionalized polyolefins, functionalized ethylene-vinyl acetate copolymers, ethylene-acrylate copolymers, ionomers, polyalkylene oxide polyester block copolymers, derivatives of carboxymethylcellulose and blends of these polymers with polyolefins.
However, it has now been shown that these adhesion promoters generally do not produce an adhesion-based bond, especially in the polyamide/polyester system. Even if a certain adhesion is achieved in some cases, it is lost on warming or on contact with solvents, since the adhesion promoters are not sufficiently heat- and solvent-resistant. In addition, bonds of this type easily fail when subjected to shear stress due to cold flow of the adhesion promoter.